Niobium base high temperature alloys



2,838,395 NIoBIUM BASE HIGH TEMPERAT ALLOYS Thor N. Rhodiu, Jr., Wilmington, DeL,

du Pont de Nemonrs and Company, Wilmington, a corporation of Delaware No Drawing. Application November 14, 1956 Serial No. 621,995

Claims. (Cl. 75-474) assignor to E. I. Del.,

This invention relates to novel niobium-base alloys and more particularly to improved niobium-aluminumiron alloys exhibiting unusual strength and oxidation resistance under extreme high-temperature service conditions.

For an alloy to be useful as a material of construction in applications such as jet and diesel engines, atomic in applications of the type mentioned.

It is among the objects of this invention to overcome :these disadvantages of prior metallic construction materials and to provide a novel alloy composition which is particularly adapted and useful for attaining these objects. Further objects'of the invention include the provision of an improved, workable alloy composition having superior strength and oxidation resistance characteristics at'relatively high, above 1000 C. temperatures; the provision of niobium-base alloy useful in the applications mentioned and adapted to Withstand high mechanical stresses at temperatures above 1000 C.; to provide a niobium-aluminum-iron alloy which is desirably resistant to oxidation at temperatures considerably above 800 C., and in a range of l000l300 C. or higher, and which is satisfactorily ductile and readily amenable to mechanical fabrication under hot or cold working or drawing conditions, including hot swaging, hot rolling, forging, extrusion, hot pressing, etc; to provide a niobium-aluminum-iron alloy having requisite, superior hardness properties and which does not require thermal treatment to develop maximum strength at elevated temperatures; to provide an alloy composition of the type mentioned having superior fatigue, tensile and rupture properties at relatively high temperatures and which undergoes no significant permanent dimensional changes-upon subjection to prolonged exposure under extreme temperature conditions; and to provide a niobiumaluminum-iron alloy composition having unique, especially protective surface layers of reaction products and which advantageously exhibits such chemical and mechanical attributes as desired adherence, nonpermeability, dimensional stability, nonvolatility and minimum film thickness when exposed to corrosive atmospheres at high temperatures. Other objects and advantages of the invention will be evident from the ensuing detailed description.

These and other objects are realized by the alloys of this invention which contain as essential ingredients at least 55% by weight of niobium, about 1-20% by weight of aluminum and about 125% by weight of iron. In combination with these elements, and to impart to the ice alloy certain desired characteristics, such as the properties of protective oxide scale or the. special metallurgical response of the alloy to working, such as for example, heat treatment or fabrication, there can be added from 0-35% by weight of one or-more of the elements cobalt, nickel, tungsten and zirconium, from 0-5% by weight of one or more of the elements beryllium, manganese, molybdenum, silicon, thorium and vanadium, and from 02% by weight of one or more of the elements boron, carbon, calcium and cerium. When employing mixtures of two or more of said added elements, the total quantity used from said 0-5% range should not exceed 15% by weight; and when from said 0-2% range, the total should not exceed 5% by weight.

In a more specific and preferred embodiment, my novel alloy composition contains from about 55% to by weight on niobium, from 4% to 20% by weight of aluminum, and from 4% to 24% by weight of iron, with the amounts of added elements present being in the following ranges and total amounts: 0-20% by weight of one or more of said elements nickel, tungsten, zirconium and cobalt, the total of this group not to exceed 35%; 0-5% by weight of one or more of said elements beryllium, manganese, molybdenum, silicon, thorium, and vanadium, the total of this group not to exceed 15%; and from 02% by weight of one or more of said elements boron, carbon, calcium and cerium the total of this. group not to exceed 5%.

My improved alloys can be prepared in accordance with conventional procedures and through recourse to known melting and casting techniques. Thus, the individual metals can be melt-cast together and the melt allowed to cool and solidify into a desired shape. The melting operation can be carried out in an arc melting furnace provided with consumable or nonconsumable electrodes, or by subjecting the charge to induction heating in a skull or crucible type of container. One useful form of arc melting furnace comprises that having an integral, water-cooled copper crucible in which the charge can be melted and solidified, such as that described by W. Kroll in Transactions of the Electrochemical Society, vol. 78, pages 3547, 1940. Alternatively, a compressed, consumable arc electrode type melting furnace can be employed, such as described in U. S. 2,640,860 to S. A. Herres, as can the combination of a nonconsumable and consumable electrode type of double-melting furnace described in US. 2,541,764, to S. A Herres. A continuous feed type of furnace can also be used, such as described in U. S. P. B. Report 111,083. Whatever type furnacing means is employed, care should be exercised in the melting and casting operation to protect the molten metal from normal atmospheric contamination through contact with oxygen, nitrogen, etc. This can be prevented by conducting the operation under a vacuum or an atmosphere of an'inert gas, such as argon, helium, etc. 7 The individual metals charged to the melting furnace can be in any desired form, e. g., powder, granular, shot, wire or sponge, and should be of commercially acceptable purity to insure production of a satisfactorily pure alloy product. The cast material obtained will consist of a workable metal having excellent strength and oxidation resistance at high temperature, and will be eminently suitable for use as a material of construction in high temperature equipment designed to operate at temperatures beyond the limits of present .equipment constructed of the best high-temperature alloys.

Advantageously, my alloys will exhibit high strength at temperatures ranging from 1000-1300 C. or higher, at which temperatures other high temperature alloys lose strength, become plastic or melt; will be characterized by especially protective layers of reaction products on or below the metal surface consisting of compounds of the alloy with oxygen, nitrogen, hydrogen, carbon, sulfur or halogens or compounds thereof present in the atmosphere; and will be found to have been adjusted to produce especially protective surface layers containing combinations of the compounds mentioned with themselves, such as spinel oxides or with each other, such as mixed oxides-nitrides to provide very high resistance to deleterious attack of the alloy by the surrounding gases. The data given below demonstrates the properties of the alloy in respect to hightemperature oxidation resistance. Their performance with reference to a balance between oxidation resistance and fabricability is determined by the relative proportions of the alloying elements. Since these two properties tend to oppose each other, the ranges in composition given were chosen on the basis of establishing an optimum compromise between them.

To a clearer understanding of the invention, the following specific examples are given. These are only illustrative and are not to be considered as limiting the scope and underlying principles of the invention. As will be noted, many of the aluminum-modified niobium-base alloys given are characterized by unusually high niobium content, and in the preferred 55-80% range. In addition, the protective films characteristic of oxidation at 1000 and 1200 C. often contain relatively little niobium oxide but are usually substantially enriched in aluminum oxide. This inversion in the niobium-aluminum ratio between the alloy and the protective films is especially unique for these alloys and comprises a novel and outstanding feature of my invention.

EXAMPLE I A granular mixture of 71% by weight niobium, 19% by weight aluminum, and 10% by weight iron was charged into a water-cooled, copper crucible of an arc melting furnace of the type described above and the metals heated under an atmosphere of helium to effect complete fusion of the metal charge. When the charge became liquefied the furnace was turned off and the melt was allowed to cool in the inert atmosphere, discharged from the crucible and was tested for resistance to high temperature oxida tion in the following manner:

A coupon was cut from the as cast button and heated at 1000 C. and 1200 C. for 24 hours in a helium atmosphere. The sample was then heated to 1000 and 1200 C. in a recording thermobalance in flowing air for 24 hours. Oxidation rates were followed by making continuous measurements of weight change while the sample was at a controlled temperature without interruption of the test during the 24-hour period. Nonvolatility of surface compounds under these test conditions was also determined by mcasuring no weight change when exposed to pure helium.

Upon termination of the oxidation test, the sample was cooled and the protective character of the surface layers determined by metallographic examination and chemical analysis. In addition, the effect of oxidation on the metal alloy itself was examined by the same methods. It had an oxidation rate of 0.03 mg./sq. cm./hr. after 24 hours at 1000 C. and a rate of 0.09 tug/sq. cm./hr. after 24 hours at 1200 C. A specimen of pure niobium metal, subjected to the same test had, in contrast, an oxidation rate of 22.0 mg./sq. cm./hr. after 24 hours at 1000 C. and 68 mg./sq. cm./hr. at 1200 C.; and, in some cases, was completely converted to the oxide after treatment at 1000 C. and 1200 C. The specimen of this example, on the other hand, was covered with a very thin, especially adherent protective oxide layer which corresponded to less than 0.03% conversion of the metal at 1000 C. and less than 0.08% conversion at 1200 C. This layer showed outstanding resistance to spalling when the specimen wns heated to 1000 C. and 1200 C. and then cooled to room temperature. Upon forging and machining the remaining casting into a nozzle element and employing EXAMPLE II An alloy was prepared in the same manner as in Example I except that its composition was 71% by weight niobium, 9% by weight aluminum, 20% by weight iron.

After testing as described in Example 1, its oxidation rate was 0.03 mg./sq. cm./hr. after 24 hours at 1000 C. and 0.20 mg./sq. cm./hr. after 24 hours at 1200 C. This corresponded to less than 0.05% and 0.10% conversion of the metal at 1000" C. and at 1200 C. respectively. The adherence and coherency of the oxide film in this alloy was found to be exceptional particularly upon heating and cooling from 1200" C. or from 1000 C. to room temperature.

EXAMPLE III An alloy was prepared as described in Example I except that its composition was 82% by weight niobium, 6% by weight aluminum, 10% by weight iron, and 2% by weight of boron. A coupon was cut from the cast material and upon heating and testing as in Example I had the properties shown in Table I below.

EXAMPLE IV An alloy was prepared as in Example I except that its composition was 75% by weight niobium, 10% by weight aluminum, by weight iron, and by weight nickel. Upon subjecting a coupon cut from the as-cast material to the test described in Example I, it had characteristics as shown in Table I below.

EXAMPLE V An alloy was prepared as described in Example I except that its composition comprised 56% by weight niobium, 20% by weight aluminum, and 24% by weight of iron. Upon testing as in Example I, it had the properties shown in Table I below.

EXAMPLE VI An alloy was prepared as described in Example I except that its composition comprised 56% by weight niobium, 10% by weight aluminum, 9% by weight iron and 25% by weight of nickel. Its characteristics are shown in Table I below.

EXAMPLE VII An alloy was prepared as described in Example I except that its composition comprised 60% by weight niobium, by weight aluminum, by weight iron, and 5% by weight of molybdenum. Its characteristics are shown in Table I below.

EXAMPLE VIII An alloy was prepared as described in Example I except that its composition comprised 56% by weight niobium, 14% by weight aluminum, 10% by weight iron, 15 by weight cobalt, 4% by weight molybdenum and 1% by weight cerium. Its characteristics are shown in Table I below.

Table I (Oxld/ntlon 171311;? Adh mg. sq.c111. r. erence Example of Oxide Scale 1,000 C. 1,200 G.

Control 100% Nb 22 68 Very Poor. I-71% Nb, 19% Al, 10% Fe- 0. 03 0. 09 Good. II-7l% Nb, 9% Al, 20% Fe.. 0. 08 0.20 Excellent. III-82% Nb, 6% Al, 10% Fe, 2% 0.09 0. 34 Good. IV-75% Nb, 10% Al, 5% Fe, 10% Ni. 0. 09 0. 15 0. V-56% Nb, 20% Al, 24% Fe O. 04 0. 08 Excellent. I I-56% Nb, 10% A], 9% Fe, 25% N1. 0. 01 0. 04 I Do. VINII-% Nb, 15% Al, 20% Fe, 5% 0.08 Fair.

1 o. VIII-56% Nb, 14% Al, 10% Fe, 15% 0. 01 0. 05 Excellent.

00, 4% Mo, 1% Ce.

As noted, the alloys of this invention are useful as materials of construction in all applications requiring strength and a corrosion-resistant metal. Hence, while particularly useful in high-temperature equipment which must operate above 800 C., such as jet engine parts, nuclear reactors, gas turbine parts, etc., my novel alloys due to their outstanding properties, including nonbrittleness and adaptability for successful fabrication by hot swaging or rolling, forging, extrusion or hot pressing, are not restricted to such applications nor to any particular equipment described or referred to herein.

When recourse is had to the use of cobalt, nickel and zirconium in combination with the contemplated amounts of niobium, aluminum and iron, from about 1-35% by weight of one or more of the elements cobalt, nickel, tungsten and zirconium can be employed; when beryllium, manganese, silicon, thorium, and vanadium are used, amounts ranging from 15% by weight can be employed in conjunction therewith, the total of this group not to exceed while from 0.l-2% by weight of one or more of the elements boron, carbon and cerium can be used with the niobium, aluminum and iron, the total of this group not to exceed 5%.

Although I prefer to employ metals exhibiting relatively high purity, some variance in purity properties can be tolerated. Thus, the alloys of the examples and those tested were prepared from commercially available niobium, aluminum and iron containing less than 1% incidental impurities. Commercial niobium usually contains tantalum (in amounts up to 5%) which is difiicult to detect and separate. Therefore, the niobium used herein may contain small amounts (0.1 to 5.0%) of tantalum, as well as iron, oxygen and possibly silicon as impurities. Elimination of certain of these impurities (silicon, oxygen) or enhancement of others (tantalum, iron) may improve oxidation resistance significantly.

Since many changes and modifications can be made in the invention without departing from its underlying principles, it will be understood that it is not restricted to the above detailed description, but only as defined in the appended claims.

I claim:

1. A niobium-base alloy containing as essential ingredients at least 55% by weight of niobium, from l-% by weight of aluminum and from 1-25% by weight of non.

2. An oxidation-resistant niobium-base alloy composition containing from about 55-80% by weight of niobium, from about 4-20% by weight of aluminum and from about 1% to by weight of iron, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

3. A niobium-base alloy containing at least 55% by weight of niobium, from 120% by weight of aluminum, from l-25% by weight of iron, and in combination therewith from 0-35% by weight of an element selected from the group consisting of cobalt, nickel, tungsten and zirconium, from 0-5% by weight of an element selected from the group consisting of beryllium, manganese, molybdenum, silicon, thorium, and vanadium, and from 0-2% by weight of at least one element selected from the group consisting of boron, carbon, calcium and cerium.

4. An oxidation-resistant niobium-base alloy composition containing from about 80% by weight of niobium, from about 4-20% by weight of aluminum, from about 1% to 25 by weight of iron, and from l25% by Weight of nickel, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

5. An oxidation-resistant niobium-base alloy composition containing from about 55-80% by weight of niobium, from about 4-20% by weight of aluminum, from about 1% to 25 by weight of iron, and from l-20% by weight of cobalt, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

. 6. An oxidation-resistant niobium-base alloy composition containing from about 55-80% by weight of niobium, from about 4-20% by weight of aluminum, from about 1% to 25% by weight of iron, from 125% by weight of nickel, from 0.1-5% by weight of tungsten, from 1-20% by weight of cobalt and from 0.12% by weight of cerium, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

7. An oxidation-resistant niobium-base alloy containing about 71% by weight of niobium, about 19% by weight of aluminum and 10% by weight of iron.

8. An oxidation-resistant niobium-base alloy containing by weight about 56% niobium, about 10% aluminum, about 25 of nickel, and about 9% iron.

9. An oxidation-resistant niobium-base alloy containing by weight about 56% niobium, about 14% aluminum, about 10% iron, about 15% cobalt, about 4% molybdenum and about 1% cerium.

10. A niobium-base alloy containing at least 55% by weight of niobium, from 120% by weight of aluminum, from 125% by weight of iron, and in combination therewith from 0-35% by weight of at least one element selected from the group consisting of cobalt, nickel, tungstem and zirconium, the total of the elements of this group not to exceed 35%; from 0S% by weight of at least one element selected from the group consisting of beryllium, manganese, molybdenum, silicon, thorium, and vanadium, the total of the elements of this group not to exceed 15%; and from 0-2% by weight of at least one element selected from the group consisting of boron, carbon, calcium and cerium, the total of the elements of this group not to exceed 5%.

References Cited in the file of this patent Initial Investigation of Niobium and Niobium-Base Alloys, Saller, Stacy, Porembka, AEC, BMI1003, May 23, 1955. 

9. AN OXIDATION-RESISTANT NIOBIUM-BASE ALLOY CONTAINING BY WEIGHT ABOUT 56% NIOBIUM, ABOUT 14% ALUMINUM, ABOUT 10% IRON, ABOUT 15% COBALT, ABOUT 4% MOLYBDENUM AND ABOUT 1% CERIUM.
 10. A NIOBIUM-BASE ALLOY CONTAINING AT LEAST 55% BY WEIGHT OF NIOBIUM, FROM 1-20% BY WEIGHT OF ALUMINUM, FROM 1-25% BY WEIGHT OF IRON, AND IN COMBINATION THEREWITH FROM 0-35% BY WEIGHT OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF COBALT, NICKEL, TUNGSTEN AND ZIRCONIUM, THE TOTAL OF THE ELEMENTS OF THIS GROUP NOT TO EXCEED 35%, FROM 0-5% BY WEIGHT OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF BERYLLIUM, MANGANESE, MOLYBDENUM, SILICON, THORIUM, AND VANADIUM, THE TOTAL OF THE ELEMENTS OF THIS GROUP NOT TO EXCEED 15%, AND FROM 0-2% BY WEIGHT OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF BORON, CARBON, CALCIUM AND CERIUM, THE TOTAL OF THE ELEMENTS OF THIS GROUP NOT TO EXCEED 5%. 